Figure 2
(a) Example of indicatrix orientations at temperature T = 140 K. Two types of indicatrix orientations corresponding to [100] (magenta) and [010] (blue) can be distinguished. (b) Temperature dependence of the birefringence calculated for small regions marked by the white framed rectangular in (a). Below this temperature, the abrupt change of Δn is a signature of a different phase transition. The inset in Fig. 2b is the difference in birefringence between the [100] and [010] orientations. While the transition near 210 K is well visible, slightly above this temperature, some traces of the Δn might be incompatible with the tetragonal symmetry.
